async-test.rb

#!/usr/bin/env ruby

require "async"

class Global
  class << self
    attr_accessor :counter
    def counter
      @counter ||= 0
    end
  end
end

Async do |task|
  1000.times do
    task.async do
      val = Global.counter
      sleep 0.0001
      val += 1
      Global.counter = val
    end
  end
end

puts Global.counter